1. Field of the Invention
This invention relates to an optical device for extracting information carried in sidebands or tones within a composite signal exhibiting multiple types of signal modulation and, more particularly, to an optical device adapted to extract optically either a frequency shift keying (FSK) modulated signal from a composite FSK/IM (intensity modulated) signal or a sub-carrier modulation (SCM) signal from a composite SCM/IM signal.
2. Description of the Related Art
Data networks and optical networks are converging in an attempt to evolve the optical Internet of the future. As such, future optical data communication links are likely to operate on burst mode traffic as opposed to the regular, periodic traffic found in conventional time-division-multiplexed (TDM) networks. In order to handle the burst mode traffic efficiently, asynchronous connections will replace synchronous connections between network nodes in the optical data networks such as packet switched networks.
One promising technique for implementing these packet switched networks involves putting the data arranged in an internet protocol (IP) format or in any other kind of packet format directly onto an optical wavelength division multiplexed (WDM) channel. This technique avoids any involvement with the Synchronous Digital Hierarchy (SDH) and Asynchronous Transfer Mode (ATM) transport layers. IP-over-WDM is supported by an optical multi-protocol label switching protocol (MPLS) formerly known as MPλS protocol, an optical multi-protocol wavelength switching protocol and now part of Generalized Multi-Protocol Label Switching (GMPLS) particularly with reference to the Lambda-Switched Capable (LSC) level.
Generalized Multiprotocol Label Switching (GMPLS) is the next-generation implementation of Multiprotocol Label Switching (MPLS). GMPLS extends the functionality of MPLS to include a wider range of label-switched path (LSP) options for a variety of network devices. GMPLS labels and LSPs can be processed at four levels. The levels are Fiber-Switched Capable (FSC), Lambda-Switched Capable (LSC), Time Division Multiplexing Switched Capable (TDM), and Packet-Switched Capable (PSC).
According to the GMPLS protocol at the LSC level, wavelength-switched channels can be established in a similar way as label-switched paths in the conventional MPLS protocol. In GMPLS systems, packets can be marked with an optical label that can be swapped in every network node. Thus, end-to-end optical paths can be set up along which the data can be routed transparently through the network. Currently, several label marking techniques for optical label switching have been investigated and two techniques of interest herein are Subcarrier Multiplexing (SCM) and Frequency Shift Keying (FSK).
In both label marking techniques considered for optical label switching, label data is modulated as two optical frequencies (generally called optical tones) symmetrically distributed about a central optical frequency fc (generally known as the channel frequency or carrier frequency). The payload of packet data is modulated, generally by an intensity modulation technique such as amplitude shift keying (ASK), onto the optical signal in the region at or around the central optical frequency. An exemplary composite optical signal bearing the payload in the central portion of the composite signal and the label in the sidebands of the composite signal is shown in FIGS. 1 and 2 for the SCM and FSK label marking techniques, respectively, on an IM payload.
At each network node, it is necessary only to process the optical label that encapsulates all switching and routing information for the packet, while the payload portion of the optical packet remains unaffected in the optical domain. Processing has been carried out in exemplary systems by using optical filters tuned to the label and payload optical frequencies. This technique however has several disadvantages, namely, the loss of optical power, the duplication of components for each operation, and the need for tuning. Optical power is lost because the incoming signal is split between two paths—one for the label processing and one for the payload processing. The processing operation in each path necessarily discards the unwanted signal (label or payload) while extracting the wanted portion. Tuning to the channel frequency must be maintained at the correct optical frequency, whether for the FSK tones or the SCM sidebands. If the center frequency is changed to a different WDM channel, the optical filters must be retuned in a difficult and slow process.
Fiber Bragg gratings have been suggested for use in label extraction by exploiting the reflection and transmission functions of the gratings. This approach is considered lossless in the sense that the fiber Bragg grating has two output ports that one can match with the label optical frequency and the payload frequency. But the suppression ratio depends mainly on the manufacturing process. Current grating designs provide adequate, but not ideal, suppression ratios. It should be noted that grating designs have a finite suppression ratio which can be adequate for detection. This suppression can be, for example, −30 dB to −45 dB and depends on the manufacturing process, materials used and other design parameters.
The techniques discussed above do not exhibit efficiency in separating labels from packet payloads in a lossless way and they do not yield adequate extinction ratios between the two optical label tones in the case of FSK. Whether using optical filters or Bragg gratings to realize the label and payload separation function, one is required to tune the payload and label separation devices precisely to each new WDM channel. When the WDM system requires different channel spacing, the payload and label separation devices must be retuned to the new channel frequencies even though the spacing of FSK tones or SCM sidebands remains the same regardless of WDM channel spacing as with ITU channels, for example. There is no known technique for simply realizing a device that exhibits a periodic filtering function that would filter the label tones without retuning to the new WDM channel frequency.